A geothermal heat pump or geoexchange system is actually comprised of three separate systems:

-- the ground loop for extracting or rejecting heat into the ground to warm or cool the building as needed
-- the heat pump itself that concentrates the heat
-- the delivery system in the form of air ducts or a radiant floor system.

A heat pump merely transfers the heat in the ground that already exists between the earth and the building. This gives the heat pump the energy efficiency advantage of using a small amount of electricity to tap into existing heat rather than warming cold air by burning fossil fuels, as is done by natural gas forced air heat.

Just a few feet below ground the temperature is about 55 degrees Fahrenheit, considerably warmer than winter temperatures in much of the U.S. Heat pumps tap into this source of warmth that is basically stored solar heat. A heat pump operates on the same basic principles as that of the ordinary refrigerator, except a refrigerator only cools, whereas a heat pump alternatively heats or cools.

The heat pumps basic components include a heat exchanger, compressor, reversing valve, air condenser and an expansion valve/metering device. In the heating cycle, environmentally-friendly anti-freeze solution from the ground loop enters the heat pump and passes through a heat exchanger where latent heat energy is transferred to a refrigerant, which is piped to the compressor via the reversing valve. The compressor raises the temperature of the refrigerant to approximately 140 degrees Fahrenheit.

After the compressor the refrigerant passes through the domestic hot water heater to warm the building's water supply. Next, the system pushes the refrigerant to the next stage where it is used to heat the building as it gives up its latent energy and re-condenses in a refrigerant-to-air or a refrigerant-to-water heat exchanger.

(Refrigerants are one of the most efficient chemicals for absorbing and transferring heat energy back and forth between structures or the outdoors. Historically, R-22 has been the preferred refrigerant of the geothermal heat pump industry., but Congress mandated the discontinuation of R-22 production by the year 2010 because of concerns about its affect on the ozone layer. Ground loop systems purchased today should use the replacement, R-410a, which can not be used in earlier systems.)

From there the refrigerant passes through an expansion/metering device before it returns to the water/refrigerant heat exchanger connected to the ground-loop. The ground loop circulating pump and the actual heat pump each use the equivalent of ten 90 watt light bulbs in terms of energy consumption per ton of heat pump capacity.

In the summer, when temperatures can reach triple digits, geothermal systems take advantage of the ground acting as a heat sink. The cooling process is essentially the reverse of the heating cycle in that heat is drawn out of the building and collected in the air condenser/heat exchanger. The reversing valve allows the owner to switch between heating and cooling by changing the direction of the refrigerant flow. In the cooling cycle the refrigerant passes through the air condenser before going through the compressor.

Heat pumps are electrically powered systems, so if the electricity is generated by renewable energy source such as wind or solar, the heating/cooling could be completed without any burning of fossil fuels.

Ground Loop Options

Ground loops can be open, in which fluids are injected into the ground after they are used for heating or cooling, or closed loops. Because open loops often require special permits and may not offer the same tax incentives as closed loops, this article focuses on closed loops.

Ground loops are categorized as horizontal, vertical, pond and slinky. Vertical loops can reach greater depths but at greater expense, while horizontal loops require more land area because the ground temperature is less stable at shallow depths. A pond loop is the most cost effective because it does not require any drilling or trenching, which is the one of the most significant installation expenses. A slinky loop substitutes for a horizontal loop if the available area is not sufficient and a vertical loop is too expensive or difficult to install.

For a typical single home installation of a three ton geothermal heat system, approximately 9000 square feet of area is required for a horizontal loop, while a vertical loop requires three 200 foot deep loops. Technicians can generally install any type of ground loop system within a few days. However, some subsurface conditions, such as shallow bedrock which can prohibit drilling, prevent or make the installation of geothermal heat pumps difficult.

The loops use plastic pipes that won't corrode, and the piping system will not affect the use of the land above as a lawn or garden. The heating unit is typically located inside the house and takes up about the same amount of space as a forced air heating unit.

Curt Sommer is a renewable energy consultant based in West Linn, OR and can be reached through his website.